Elucidation of the cellular/molecular mechanism(s) of caloric restriction (CR), which inhibits the development of a variety of spontaneous and experimentally-induced tumors in rodents, may provide important clues for human cancer prevention. We are using a p53-knockout transgenic mice as an in vivo model to explore the mechanisms underlying the anti-tumor effects of CR. The p53 gene is the most commonly identified mutated gene in human tumors. Recent evidence suggests that the wild-type p53 gene product protects cells against mutations by mediating an arrest of the cell-cycle in response to DNA damage, facilitating repair of damaged DNA and preventing fixation of mutagenic lesions that can lead to neoplasia. Mice with the p53 gene knocked-out by homologous recombination develop normally but have increased susceptibility to spontaneous tumor development, with approximately 100% tumor incidence by 6 months of age in untreated homozygous p53-deficient mice. We have shown that CR markedly decreased the incidence and increased the latency of spontaneous tumor development in these mice. Cellular and molecular studies on tissues collected serially from wild- type, homozygous and heterozygous p53-deficient mice fed ad libitum or CR- treated, are currently in progress. We have also begun analyzing tissues for differences in p53, ras, fos/jun and mdr expression using Northern blot analysis. We are also conducting a 2-stage skin tumorigenesis experiment with wild-type and heterozygous p53-deficient mice to determine the stage in the carcinogenesis pathway in which CR is exerting its effects and to further explore the mechanisms underlying the anti-tumor effects. The effect of caloric restriction in the presence and absence of the p53 gene on the development of papillomas and the progression of papillomas to carcinomas will be evaluated. Finally, we have established in vitro embryonic fibroblast cell lines isolated from wild-type, homozygous and heterozygous p53-knockout mice to facilitate the evaluation of potential intermediates of the tumor- inhibitory effects of CR.